The front end structure of an automotive vehicle is designed to provide visual appeal to the vehicle owner while functioning as an energy absorbing structure during frontal and offset crashes. The size, shape and construction of the front end structure contribute to the ability of the front end structure to attenuate the crash pulse and restrict intrusions into the operator's cabin of the vehicle. It is important to design a front end structure to absorb crash energy through the frame components. To that extent, a significant amount of effort by vehicle engineers is devoted to designing the vehicle frame to crush in a controlled manner while absorbing a maximum amount of energy. One of the goals in the design of vehicle frame structure is to provide better engagement and absorption of energy during a collision. The major components in absorbing energy in frontal as well as rear impacts are the frame rails.
Vehicle frames typically include an upper rail and a generally vertically spaced lower rail. Preferably, the upper rail joins the lower rail, such as at the forwardmost portion of the vehicle frame, to define an integrally connected automotive frame structure. The structural joint connection between the vehicular upper and lower structural member is conventionally designed as a solid connection which provided good structural integrity in all directions. One approach to the management of crash energy is to reduce the structural efficiency of the joint between the upper and lower rail members in the fore-and-aft direction to allow a “break away” while experiencing a safety load condition. While effective management of crash energy will include crash triggers in the front end structure, such as the horn connecting the front bumper to the frame rails, and by properly designing the structural joint between the upper and lower rails. The management of crash energy includes the direction and tuning of the load path along which the crash energy is directed. While the formation of the upper and lower rail members is preferably accomplished through hydroforming techniques which forms the upper and lower rails as tubular members, the upper and lower rails can be formed of any material or any construction technique, including stamped and roll-formed vehicular body structures.
The package constraint for the placement and design of the front rail system can present a problem with respect to the energy management function of the front end. In automotive frame configurations in which the package constraint forces the rail to bend downwardly as the horn section approaches the bumper beam, which will allow the automotive frame to meet a 16-20 inch bumper height requirement, a front impact exerts an offset eccentricity between the center of gravity at the bumper and the center of gravity of the subframe attachment. This offset eccentricity can result in a substantial external applied bending at the center of gravity of the front rail section, which can be a large percentage of the bending capacity of the front rail section. Thus, this external applied bending takes away from the section capability to manage the normal buckling and folding stresses due to axial collapse of the horn section of the lower frame rail member. This eccentricity of the frame configuration can result in a premature downward bending of the horn section at the onset of any axial crash.
One approach to resolving this package and loading constraint problem is to reinforce the rearward half or third of the horn section rail length closest to subframe attachment, at lower side of the section where buckling stresses are highest, resulting in a corresponding increase in the bending capacity of the horn section. Typically, this reinforcement is provided in the form of a vertically oriented flange extending downwardly from the horn section. With appropriate structure, the crushability of the horn section can be accomplished effectively to direct the load path for the crash energy into the frame rails.
Mid rail structure is cumbersome due to the various components that are typically attached to the lower frame rails at the crash zones. Subframe, shock tower, engine, transmission in case of rail mounted power train are a few of the components that constrain the controlled crashing of the mid rail structure. The mounting of the engine on the mid-rail structure creates a significant difficulty in managing crash forces. Straight vehicle rails, whether they are parallel to the longitudinal axis of the vehicle or are slightly tapered outboard and/or downward, tend to remain straight because of the higher peak load to induce axial buckling and, therefore, shift the energy management to the backup structure through a bending of the upper and lower elbows of the backup rails. Such frame rail designs do not fully utilize the energy management capability of the mid-rail zones. If triggered correctly, these frame rail designs might be capable of yielding a single bending hinge prior to collapsing the backup structure. The introduction of an engine mounting bracket over the mid-rail zone effectively eliminates any opportunity to manage energy over this mid-rail zone.
U.S. Pat. No. 3,794,348 granted to Hans Fischer et al on Feb. 26, 1974, discloses a frame structure for an automobile that is intended to manage energy in the event of a collision. More specifically, a frame member is made by assembling together four edgewise-abutting longitudinal plates, including a flat upper plate, a flat lower plate and two side plates. The end region of the frame member has interruptions in the welding of both the lower and upper plates to weaken the weld so as to buckle the end region under impact.
The front end subframe structure in U.S. Pat. No. 6,880,663, issued to Hiroyuki Fujiki, et al on Apr. 19, 2005, is used to support a power unit at a support point with a weak section formed on the front side of the support point toward the front of the vehicle so as to be able to bend in case of a frontal impact. U.S. Pat. No. 6,938,948, granted on Sep. 6, 2005, to Troy Cornell, et al discloses a full vehicle frame with a subframe mounted engine where the vehicle frame and the subframe are designed to have crushable junctures that allow the engine cradle to move downwardly and rearwardly in a frontal impact situation.
It would be desirable to provide a frame rail configuration that would enhance the management of crash forces by inducing outboard structural thinning at the mounting of the engine and an inboard material thinning pocket at the end of the engine mounting to affect a first crash hinge in the frame rail and a V-necking at the middle of the frame rail to affect a second sequential bending hinge in a crash mode.